The present invention relates to thermowells of the type used in temperature sensors used in industrial process control and monitoring systems. More specifically, the present invention relates to measurement of vibration of such thermowells.
Industrial process sensors and transmitters are used to sense various characteristics of fluid flowing through a conduit, or contained within a vessel. The transmitters sense process parameters such as differential pressure, line pressure, temperature, and pH.
Temperature sensors such as thermocouples, resistance temperature detectors or infrared sensors in process applications are usually protected by a metal or ceramic sheath known as a thermowell. The sensor electrical leads are isolated from each other and from the metal sheath and metal parts through some kind of isolating material. The assembly consisting of the sensor, sensor electrical leads, sensor sheath, isolating material and installation fittings is called a sensor assembly.
The sensor leads are connected to an electronic circuit that reads the sensor signal and convert it to a temperature reading. This electronic circuit can reside in an input electronic card of a control, monitoring or safety system or in a transmitter. Transmitters are usually installed relatively close to the temperature sensor.
The transmitter converts the sensor signal to a temperature measurement value and transmits the signal to a remote recipient such as a control, monitoring and/or safety system. The temperature value can be transmitted through different types of signals and media. It can be converted into an analog standard value such as 4 to 20 mA or through digital protocols such as HART®, Fieldbus, Profibus, DeviceNet, Modbus, Ethernet, etc. The transmitting media can be via wires, fiber optic, infrared or RF.
Temperature sensors used in industrial processes are typically fitted with a primary seal such as a thermowell. Thermowells are used to provide an additional protection to the temperature sensor. Thermowells are closed-end metal or ceramic tubes that protect temperature sensors from process pressure, erosion and corrosion. They also allow for the installing and removal of sensors without having to shut down the process. As the thermowell is a generally elongated tube, it is possible for the thermowell to vibrate and even break or crack. This may allow the release of process fluid and require an unscheduled shut down of an industrial process in order to repair any damage.